The charged particle microscope that is represented by a scanning electron microscope can observe a sample at a high resolution in a nanometer order.
Recently, accompanied by minuteness of the process rule of the semiconductor device, it is required to control a process defect in a nanometer order that occurs during the manufacturing process in terms of the yield. Therefore, a high resolution scanning charged particle microscope technology that uses an immersion lens is applied to a process monitoring apparatus.
In the meantime, the process monitoring apparatus is used in a manufacturing process, and requires a high throughput automatic observation. As a method for improving the throughput, a method that deviates the trajectory of the primary charged particle beam using a deflector lens to move the observation portion (hereinafter, referred to as an image shift). Since the image shift is performed by shifting the trajectory of the primary charged particle beam from an axis of the lens, the resolution is degraded due to the increase of off-axis aberrations. Therefore, the image shift is not suitable for the movement of the visual field in a millimeter order. However, the influence of the degradation of the resolution is small in the movement of the visual field of a micrometer order. Therefore, the image shift is largely used for a process monitoring apparatus that frequently moves the visual field of a micrometer order.
Generally, a deflector unit that is used for image shift is configured by upper deflector and lower deflector. The upper deflector deflects the trajectory out of the axis and the lower deflector deflects the trajectory so as to be returned, so that the beam passes through an axial center of an objective lens, thereby reducing the off-axis aberrations. The ratio of the strength between the upper deflector and the lower deflector (hereinafter, an upper and lower ratio) is affected by the rotation of the primary charged particle beam by the above-mentioned objective lens so that the condition where an optimal resolution is obtained is changed by the deflection distance of the image shift. Therefore, it is required that the upper and lower ratio is reset according to the change in deflection condition of the image shift or it is required to deflect the trajectory so as to be the trajectory of the primary charged particle beam having an optimal upper and lower ratio using a deflector other than the image shift deflector. Further, since the process monitoring apparatus used in the manufacturing process requires the automatic observation, it is required to automatically set the above-mentioned condition in accordance with the deflection distance of the image shift.
Therefore, adjustment data shown in FIG. 2 is obtained. Referring to FIG. 2, when a fixed value is used, the upper and lower ratio of the image shift is stored by associating an adjustment value, when the shift of an optical trajectory that is changed by the change in the image shift is corrected by using an optical trajectory adjuster with a deflection distance of image shift. Since dynamic focus control is controlled by a single focus system, the upper and lower ratio of the image shift may be associated only with the image shift.
Further, as a technology that automatically sets the trajectory of the primary charged particle beam that acquires an optimal resolution at the time of image shift deflection, a technology that mathematizes the off-axis aberrations of the objective lens as a function of the movement amount of the scanning area and controls the deflection angle of the deflector unit according to the equation is disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 10-247465).